tests/util: Refactor UCommand and TestScenario.

Summary of changes in UCommand:
* Extend UCommand by builder methods and simplify methods in TestScenario
* Simplify code structures where possible. Add documentation.
* Store bin_path as PathBuf and util_name as String in all structs
* Remove UCommand::util and make bin_path, temp_dir private
* Rename UCommand::with_limit -> UCommand::limit

Summary of changes in TestScenario:
* Rename some parameters in TestScenario methods to be more descriptive
* Remove ucmd_keepenv, cmd_keepenv from TestScenario. Use UCommand::keep_env instead.

tests/by-util/test_users.rs

@@ -22,7 +22,8 @@ fn test_users_check_name() {
     // note: clippy::needless_borrow *false positive*
     #[allow(clippy::needless_borrow)]
     let expected = TestScenario::new(&util_name)
-        .cmd_keepenv(util_name)
+        .cmd(util_name)
+        .keep_env()
         .env("LC_ALL", "C")
         .succeeds()
         .stdout_move_str();